www.underclassblog.com — OGYouTube is a modded YouTube app with many features, such as Popup Playback & Download Videos!

Features
You can download multiple downloads in the meantime
You can watch the video on Background & Popup Window
You can watch the video while the Screen is off
You can play all qualities *140p~1440p*
You can download the video as [MP3]
You can download the subtitles
You can download any quality
Option to press back button twice to exit the video

What's New
OGYouTube (Unroot version only) now support Devices that doesn't have Google Play Services installed
Download notifications will vibrate only instead of making sound
Fixed an issue when starting the app for the first time
Fixed an issue when clicking the option menu on subscription videos
Improved the download notification on Android Nougat
Removed "Enable all qualities" since it doesn't work anymore
Minor fixes and improvements

NOTE: If you experience difficulties with downloading OGYouTube 2.6 APK from given links below, please try to login first to those file hosting before download (it's free to create an account), make sure to uncheck "Use our download manager and get recommended downloads" or similar, if any.